Process for purifying crude benzene



Oct.

E. SATTLER ETAL PROCESS FOR PURIFYING CRUDE BENZENE Filed Dec. 5, 1956 Fig. l

= gHlllllg'gIllllg /NVENTOP5 mu. snrnm m/ UILLI OETTI/YGER W/WMMA United States Patent PROCESS FOR PURIFYING CRUDE BENZENE Emil Sattler, Essen, and Willi Oettinger, Ludwigshafen (Rhine), Germany Application December 5, 1956, Serial No. 626,424 Claims priority, application Germany December 7, 1955 5 Claims. (Cl. 260-674) It is known to purify crude benzene by bringing crude benzene vapors at elevated temperature (about 320- 350 C.) and under increased pressure (about 40-50 atm.) in contact with hydrogen or hydrogen-containing gas in the presence of catalysts, whereby hydrogenation of the unsaturated impurities and a reduction of the sulphur and nitrogen compounds takes place with the formation of hydrogen sulphide and ammonia.

This process, which has become known as pressur refining, can be carried out with pure hydrogen. Alternatively, gases may be employed which contain, in addition to a considerable proportion of hydrogen, further gases, such for example as coal distillation gases, cokeoven gas or town gas. If, for example, town gas, that is to say a gas containing about 14-15% of carbon monoxide in addition to hydrogen, is employed as the source of hydrogen, this carbon monoxide is necessarily also passed over thecatalyst provided for the benzene refining. The conditions proposed for the benzene refining in regard to temperature and pressure include a partial conversion of the carbon monoxide by reaction with the steam formed in the refining chamber, carbon dioxide and hydrogen being formed. In addition, however, a part of the carbon monoxide is also converted into methane by reaction with the hydrogen; While the firstmentioned reaction, namely conversion of the carbon monoxide, is desirable in itself because it proceeds with the formation of hydrogen, the conversion of the carbon monoxide into methane is generally undesirable because it entails a considerable consumption of hydrogen. It has therefore already been proposed that the fresh town gas or coke-oven gas which is continuously introduced into the process, and which is intended to replace the hydrogen consumed by the refining process, should be subjected before being introduced into the process to a converting reaction in a particular arrangement in which substantially all the carbon monoxide content of the fresh gas is converted with the exception of a small residue. With such a method of operation, there is required in addition to the actual refining arrangement a special converting arrangement, the cost of construction and operation of which is economically tolerable only in the case of relatively large refining installations.

In accordance with another proposal, which also does not form part of the prior art, the fresh coke-oven gas saturated with steam is introduced directly into the bed of catalyst in which the treatment of the benzene vapors with the hydrogen gas takes place, at a point at which the purification of the benzene has already reached an advanced stage. This last-mentioned proposal has the advantage that no special converting arrangement is required, but it has been found that the conversion of the carbon monoxide does not take place in the desired degree.

Investigations have shown that the reasons for the unsatisfactory conversion of the proportion of carbon monoxide in the fresh gas residue is that a considerable reduction of the partial pressure of the steam takes place 2,910,517 Patented Oct. 27, 1959 ICC in the conversion zone of the refining furnace owing to the fact that the steam introduced with the saturated fresh gas is distributed over a quantity of circulating gas which is substantially ten times greater.

This disadvantage could be overcome by also saturating the circulating gas with steam before its introduction into the refining chamber, so that when the fresh gas is added in the following part of the refining process a steam partial pressure suflicient for the conversion of the carbon monoxide is present. The saturation of the circulating gas with steam has the disadvantage that the cooling arrangement succeeding the actual refining stage must also supply the cooling energy for the condensation of the very large quantities of steam, which entails an increase in the cost of construction and operation of the said cooling arrangement.

In accordance with the present invention, the aforesaid difficulties are overcome by virtue of the fact that the arrangement for introducing the fresh gas into the stream of circulating gas is so designed that the steam introduced with the fresh gas is suflicient to effect a substantially complete conversion of the carbon monoxide. The invention resides substantially in that the means for introducing the fresh gas containing carbon monoxide and saturated with steam are so designed that there is first present in the reactor a zone of increased carbon monoxide and steam concentration in which a catalytic conversion of the carbon monoxide to form hydrogen and carbon dioxide, with heating of the fresh gas by sensible heat emanating from the circulating gas, takes place before the fresh gas is combined with the circulating gas.

The arrangement according to the invention may'be designed in various ways, depending upon whether the stream of fresh gas is introduced into the catalyst bed serving for the pressure refining immediately at the commencement of the conversion, or only after the conversion reaction is completely or substantially ended.

Two constructional forms of the arrangement according to the invention are diagrammatically illustrated in the drawings.

In the constructional form according to Figure l, 1 designates the actual refining chamber, which is provided with a catalyst bed 2 which is heaped on a grate 3. The hydrogen-containing circulating gas, which has been charged with benzene vapors in an arrangement not illustrated here and which is under a pressure of about 45 atm., is introduced into the refining chamber 1, through the duct 4 at a temperature of about 320 C. The mixture of gas and vapors flows through the refining chamber from the top downwards, a temperature increase to about 360 C. taking place as a result of the exothermic reactions. The mixture of gas and vapors leaves the chamber at this temperature by way of the duct 5 and is then fed to an arrangement (also not shown here) in which the condensable fractions of the mixture, more especially the pure benzene, are separated by cooling from the incondensable fractions, that is, from the unconsumed hydrogen gas. The hydrogen gas is then again heated and charged with further crude benzene vapors and returned into the refining chamber 1.. In the case of a particular crude benzene such as is produced, for example, in coke works, about 1600 cu. m. at normal temperature and pressure of circulating gas are required per ton of crude benzene. The carbon monoxide content of this circulating gas amounts in this case to about 2%.

The fresh gas employed to replace the hydrogen consumed in the refining is passed in the form of town gas, the carbon monoxide of which should amount to about 14%, through the duct 6 by way of the valve 7 into a heating arrangement 8, in which it is heated to a temperature of about ISO-180 C. and saturated with 3 steam. The fresh gas thus heated and saturated passes through the duct 9 into a tube 10 preferably extending along the axis of the refining chamber 1, which tube terminates in spray nozzles 11 in the lower part of the refining chamber or of the catalyst charge. The spray nozzles 11 are so designed that the fresh gas flows therefrom at such a high speed that the jets of fresh gas are maintained, with simultaneous displacement of the circulating gas flowing in the same direction, until the conversion reaction in the region of these jets has ended. There thus obtains in the region of these jets a. concentration of carbon monoxide and more especially of steam which is higher than that in the remainder of the reduction chamber, so that the conversion reaction takes place within a relatively small space to a certain residual content of carbon monoxide, for example 2%. In the conversion of the carbon monoxide, a reduction of the sulphur-containing and nitrogen-containing oxygen compounds, and more especially of the nitric oxides contained in the fresh gas, also simultaneously takes place.

Depending upon the nature of the crude benzene introduced, it is possible for the refining reactions to take. place to such an extent that the temperature increase within the refining chamber reaches unfavourably high values, and more especially exceeds the temperature of 360 C., which has been found to be the optimum temperature. In such a case, a branch stream of cold circulating gas has been introduced directly into the refining chamber at a lower point through a special duct in order to avoid an undesirable temperature increase in the lower part of the refining chamber. In the present case, however, this step is unnecessary, since the heating of the fresh gas consumes heat and an advantageous temperature equalisation consequently automatically takes place. The heat required for reaching the conversion temperature emanates from the hot circulating gas which gives up its heat to the fresh gas by external contact with the conical jets of the latter.

In the embodiment of the invention as illustrated in Figure 2, there is provided within the refining chamber a special duct 14, which may extend from the bottom into the region of thegrate 3 and which is also filled with catalyst mass 15. The catalyst 15 may be the same as the catalyst 2. Alternatively, there may be employed within the duct 14 a difierent catalyst which might accelerate the conversion reactions to a greater extent than the catalyst 2. The fresh gas saturated with steam passes through the duct 10, first into a space 16 free from catalyst mass in the duct 14, which may be provided with heating means for heating the fresh gas. The saturated fresh gas thereafter passes through the catalyst bed 15, in which the conversion reaction and other hydrogenation reactions take place. The heat required for this reaction is derived from the sensible heat of the circulating gas externally washing the duct 14, and is indirectly transmitted to the catalyst mass 15 by the wall of the duct 14. After leaving the duct 14, the stream of fresh gas is combined with the stream of circulating gas in the lower part of the refining chamber.

The two gas streams then escape in common through the duct 5. In this case also, the heat consumption of the conversion balances out any temperature increases in the refining chamber.

However, it may also be expedient in some cases to cause the duct 14 to terminate a certain distance above the grate 3. In this case, the increased hydrogen partial pressure resulting from the hydrogen production of the conversion andthe already relatively high hydrogen content of the fresh gas is available in the last phase of the benzene refining at the instant when the refining reaction is to be ended.

We claim:

1. A process for purifying crude benzene which comprises introducing said crude benzene into a reaction zone in the vapor phase with a circulating hydrogen gas and treating the thus introduced benzene in said reaction zone at superatmospheric pressure and a temperature in the range of about 320 C. to about 350 C. and in the presence of a hydrogenation catalyst; preheating addi-' tional fresh hydrogen gas containing carbon monoxide and saturated with steam to a temperature in the range of about C. to about C. and passing said additional preheated gases through said reaction zone in indirect heat exchange relationship with said circulating hydrogen gas, and catalytically converting the carbon monoxide in said fresh hydrogen gas to carbon dioxide and hydrogen while maintaining said fresh hydrogen gas effectively separated from the gaseous mixture in said reaction zone; admixing the fresh hydrogen gas after said catalytic conversion with the circulating hydrogen gas and feeding the resulting admixture to said zone.

for treatment of crude benzene.

2. The process of claim 1 wherein said fresh hydrogen gas contains 14-15% carbon monoxide.

3. The process of claim 1 wherein the treatment of said crude benzene with hydrogen is carried out at 320- 360 C., and 4050 atmospheres.

4. The process of claim 1 wherein purified benzene is recovered from said admixture and the remaining gaseous residue consisting essentially of hydrogen gas is recirculated to said zone.

5. The process of claim 1 wherein said fresh hydrogen gas is sprayed into said zone thereby forming a relatively small area therein having a carbon monoxide and steam concentration higher than that in the remainder of the zone wherein said carbon monoxide is substantially completely converted to carbon dioxide and hydrogen before said fresh hydrogen gas is admixed with the hydrogen gas used for treating the crude benzene in said zone.

References Cited in the file of this patent UNITED STATES PATENTS 1,446,984 Midgley Feb. 27, 1923 1,940,650 Russell Dec. 19, 1933 2,701,267 Urban et a1. Feb. 1, 1955 2,759,799 Berg Aug. 21, 1956 

1. A PROCESS FOR PURIFYING CRUDE BENZENE WHICH COMPRISES INTRODUCING SAID CURDE BENZENE INTO A REACTION ZONE IN THE VAPOR PHASE WITH A CIRCULATING HYDROGEN GAS AND TREATING THE THUS INTRODUCED BENZENE IN SAID REACTION ZONE AT SUPERATMOSPHERIC PRESSURD AND A TEMPERATURE IN THE RANGE OF ABOUT 320* C. TO ABOUT 350* C. AND IN THE PRESENCE OF A HYDROGENATION CATALYST; PREHEATING ADDITIONAL FRESH HYDROGEN GAS CONTAINING CARBON MONOXIDE AND SATURATED WITH STEAM TO A TEMPERATURE IN THE RANGE OF ABOUT 150* C. TO ABOUT 180* C. AND PASSING SAID ADDITIONAL PREHEATED GASES THROUGH SAID REACTION ZONE IN INDIRECT HEAT EXCHANGE RELATIONSHIP WITH SAID CIRCULATING HYDROGEN GAS, AND CATALYTICALLY CONVERTING THE CARBON MONOXIDE IN SAID FRESH HYDROGEN GAS TO CARBON DIOXIDE AND HYDROGEN WHILE MAINTAINING SAID FRESH HYDROGEN GAS EFFECTIVELY SEPARATED FROM THE GASEOUS MIXTURE IN SAID REACTION ZONE; ADMIXING THE FRESH HYDROGEN GAS AFTER SAID CATALYTIC CONVERSION WITH THE CIRCULATING HYDROGEN GAS AND FEEDING THE RESULTING ADMIXTURE TO SAID ZONE FOR TREATMENT OF CRUDE BENZENE. 